Cape Town's groundwater plan targets 'impossible'

Cape Town – Key partners in the extraction of groundwater to augment the City of Cape Town's water supply believe that the city's current 100 million litres per day target is impossible.

Speaking as an audience member at a Muizenberg Festival gathering on Tuesday, Umvoto earth sciences consultancy director John Holmes explained that the total yield from groundwater extraction remains largely unknown.

Umvoto is one of the city's main partners to develop the Table Mountain Group Aquifer and Cape Flats Aquifer for groundwater extraction.

"Theoretically, it's possible to extract 100 million litres per day from that aquifer. Is that going to happen by December this year? Absolutely not," Holmes said.

"If you are thinking about the target for the whole programme, including desalination, [it] is 500 million litres a day. Essentially, we are saying [with the targets] we are going to replace the water supply of the city in six months - it is just not possible."

Holmes, an engineer, echoed the statements of his colleague, hydrologist Chris Hartnady, who said water extraction from the Table Mountain Group Aquifer is a long-term project.

Hartnady was invited to address the intimate gathering in the African Institute for Mathematical Sciences (AIMS) centre in Muizenberg under the theme of discussing Cape Town's water future.

He joined Piotr Wolski from the Climate System Analysis Group (CSAG), Jasper Slingsby from the South African Environmental Observation Network (SAEON); Kevin Winter from the environmental and geographical science (EGS) department at the University of Cape Town (UCT) and Tom Sanya from UCT's built environment department.

'We've delayed for too long'

During his presentation, Hartnady said their current expectation is to extract up to 10 million cubic meters per annum from the Table Mountain Group Aquifer, roughly 27 million litres per day.

"But implementing that in a short space of time is not going to be easy. We are not committing to any definite figures by any definite dates," he said.

"We've delayed for too long. We had five years more or less of kicking our heels and that's been [a] very frustrating experience."

Meanwhile, climatologist and hydrologist Wolski explained that South Africa's brightest weather forecasters and climate scientists forecasted in April that Cape Town would have a wetter-than-average winter.

"What happened in April this year we had a meeting of all the climate forecasting gurus in South Africa… And we couldn't agree, with all the models at hand and all the simulations, we couldn't agree [on] how will the winter develop," he said.

"And the only sort of faint agreement is that winter was going to be wetter than average. What happened is totally opposite."

'Isn't going to be enough'

Wolski said the meeting, the Winter Rainfall Outlook Forum, was attended by roughly 60 forecasters and climate scientists across South Africa.

At his turn, Slingsby, a trained ecologist, explained that up to three months of the city's water needs is lost through invasive species.

"At the moment, the existing invasive species in our primary catchments in Cape Town are using as much water as the entire Wemmershoek Dam," he said.

"So that's enough water for the city [for] roughly two to three months."

Slingsby said predictions indicate that in the next 30 years, invasive species will consume as much as the Berg River Dam, which is seven to nine months of the city's water needs.

"All the hope is on until next winter, in the hopes that it rains, but unfortunately raining next winter isn't going to be enough."

Slingsby said that research has proved that pine forestry, which is an invasive species, has only ever been an expense to the South African economy.

The costs to South Africa in terms of water is much worse, he said.

"In terms of clearing, I think it would be a hell of a lot cheaper than desalination or any of the alternatives the city is thinking of," Slingsby said.

This quote is what I find interesting. It shows just how difficult it is to understand nature and the forces that drive the weather. It explains why we should not be over critical about those that try to forecast the weather and take what they offer as a genuine attempt at trying provide us with an interpretation of what is measured.

It is a lesson in humility if you try this for yourself.

Meanwhile, climatologist and hydrologist Wolski explained that South Africa's brightest weather forecasters and climate scientists forecasted in April that Cape Town would have a wetter-than-average winter.

"What happened in April this year we had a meeting of all the climate forecasting gurus in South Africa… And we couldn't agree, with all the models at hand and all the simulations, we couldn't agree [on] how will the winter develop," he said.

"And the only sort of faint agreement is that winter was going to be wetter than average. What happened is totally opposite."

Not without straining the existing water purification plants as a result of the added salt load surely?
Of course in areas where the sewerage basically ends up in the sea anyway, it makes no difference.
Know of some on the West Coast that are already doing that.

The problem with existing Sea Water Distillation is the huge amount of energy required to completely remove ALL of the salt content.
But it is fairly easy to remove MOST of the salt with just normal filtering - it's just not fit for human consumption because of the briny taste.
There will be some corrosion long term, but not nearly as bad as with normal sea water.
It's one of those things of diminishing returns, it takes 80% of the power to remove the last 10% of salt.
But it is possible to remove 90% of the sea water with just 10% power (normal filtering).

Originally Posted by Swa

It's ignorant to think that reason and science necessarily leads to progress.

The problem with existing Sea Water Distillation is the huge amount of energy required to completely remove ALL of the salt content.
But it is fairly easy to remove MOST of the salt with just normal filtering - it's just not fit for human consumption because of the briny taste.
There will be some corrosion long term, but not nearly as bad as with normal sea water.
It's one of those things of diminishing returns, it takes 80% of the power to remove the last 10% of salt.
But it is possible to remove 90% of the sea water with just 10% power (normal filtering).

Ja know what filtered sea water tastes like after a trip to Tristan da Cunha on their supply ship. If you did not have drining water with you, you were in serious trouble.

So a possible alternative would be to filter sea water for householders to use for washing and flushing toilets, which could be collected at central points. Would only really work in close to the shore though.

Meanwhile, climatologist and hydrologist Wolski explained that South Africa's brightest weather forecasters and climate scientists forecasted in April that Cape Town would have a wetter-than-average winter.

"What happened in April this year we had a meeting of all the climate forecasting gurus in South Africa… And we couldn't agree, with all the models at hand and all the simulations, we couldn't agree [on] how will the winter develop," he said.

"And the only sort of faint agreement is that winter was going to be wetter than average. What happened is totally opposite."